1. Field of the Invention
The invention relates to an additive utilized for the desulfurization of iron or blast furnace hot metal. More specifically, the additive comprises magnesium or its alloys and a hydrocarbon material having a volatile content.
2. Description of the Prior Art
In order to reduce the overall cost of steelmaking, it has become increasingly desirable to perform a desulfurization step on the hot metal between the blast furnace and the steelmaking furnace. To accomplish this desulfurization, a variety of materials and mixtures have been used. Initially, a magnesium-impregnated form of coke was used. Soon thereafter, techniques for pneumatic injection of salt-coated magnesium granules were developed.
More recently, mixtures of magnesium granules or magnesium powder with either lime-based powders or with calcium carbide-based powders have been increasingly utilized. In the case of lime-based powders, fluorspar, calcium chloride, sodium fluoride, soda ash or similar materials often are added to the mixtures to alter the physical or chemical characteristics of the slags resulting from the desulfurization treatment. Additionally, these materials might be added to provide some marginal improvement in the efficiency of the desulfurization process per unit of magnesium consumed.
U.S. Pat. No. 4,708,737 discloses an injectable magnesium or aluminum additive for use with a molten metal. The magnesium or aluminum metal is permeated into a particulate reagent of an inorganic alkaline earth material. A particulate product may be obtained and used from this material by grinding.
U.S. Pat. No. 4,364,771 teaches the use of an injectable granular additive which is introduced in a stream of carrier gas. It is comprised of a mixture of granular magnesium base shot and granular slag material.
U.S. Pat. No. 4,209,325 discloses the introduction of magnesium with a sintered mixture of lime and a fluxing agent into the melt by injection.
U.S. Pat. No. 4,764,211 discloses the use of calcium carbide and dried coal to achieve desulfurization. Magnesium is described as an optional inclusion. The patent discusses the need to adjust the percent composition of the coal based on its volatile components.
In the case of calcium carbide-containing mixtures, a quantity of gas generating material, which is usually limestone, is often included to break down agglomerations of the carbide mixture. This increases the surface area of the carbide exposed to the metal and thereby increases the portion of desulfurization accomplished by the calcium carbide relative to the metallic magnesium or magnesium alloy.
The common approach of the prior art, save for U.S. Pat. No. 4,764,211 is the introduction of an alkaline earth metal, preferably magnesium, into the melt. This must be done under such conditions that the desulfurization takes place, but that it is done safely and efficiently. The cost of magnesium is a limiting factor on its utilization in this application, and it is a specific goal in the industry to minimize its concentration in any additive mixture. One of the simplest additives utilizing magnesium is merely its addition in combination with lime, or calcium oxide. A weight percentage of 25 to 30 percent is normal in this case. Since magnesium is a gas generating material, other gas generators such as limestone are expected to be counterproductive when utilized with lime-magnesium mixtures. Small amounts of fluorspar may be utilized to achieve a more liquid slag, but this is considered optional.
It has been disclosed that the use of natural gas as a transport medium in steel desulfurization slightly improves the efficiency of magnesium utilization, and thereby lowers the costs of sulfur removal. To explain the improved magnesium efficiency, it has been theorized that the improvement must result from the presence of carbon in the carrying gas. When injected into liquid steel, methane or natural gas decomposes into elemental carbon, which is not recognized as a desulfurizer. Hydrogen gas, which is also not recognized as a desulfurizer in these systems, is another product.
The volume of hydrogen gas generated would be twice the volume of natural gas injected, but the volume of natural gas required for powder transport is lower than in the case of transport with nitrogen or argon, because of differences in viscosity. These effects offset each other, so natural gas injection results in only slightly more gas volume, creating only slightly more turbulence in the ladle than injection with nitrogen or argon gases. Natural gas and similar combustible gases, however, present serious safety problems when used as the conveying medium.
What is lacking in the art, therefore, is an additive which will achieve improved desulfurization of molten iron through more efficient gas generation to reduce the amount of magnesium required for the task. The additive must be safe to use, in that it cannot be explosive, and easily manufactured and transported